The Flathead River is a major force in the valley, and now county officials and residents have detailed maps to help better understand where it might be headed in the future.
A study on the channel migration zone, sponsored by the Flathead Lakers, Montana Department of Environmental Quality and Montana Fish, Wildlife and Parks, looked at 24 miles of the Flathead River and attempted to map how it might behave over time, according to Karin Boyd of Applied Geomorphology Inc.
Boyd presented the study findings during a public meeting on Dec. 7.
By using 183 measurements of historical river migration, Boyd said the study showed possible future river movement for the next century. On some of the maps, the researchers provided buffer zones of where they think the banks could be headed.
Similar studies have been performed on the Big Hole, Yellowstone and Clark Fork rivers, Boyd said.
Running from Old Steel Bridge in Columbia Falls to the mouth of Flathead Lake, the studied branch of the Flathead River has already had a dynamic history, she noted.
Aerial photos and maps from as early as 1958 showed how the river has naturally migrated; in some places, the banks moved an average of more than 400 feet in 100 years, Boyd said.
Migration has its benefits, she noted. When river channels move, they open up new sandbars, which are colonized by new plant life and help maintain the ecology. Also, shifting banks add minerals to the riverbeds, creating fertile habitat for fish species.
“When rivers move around, it gets messy and that creates complexity,” Boyd said.
Problems also arise when rivers shift course. The most notable is the threat to property investments. It can also create a headache for bridge engineers if the river decides to start undermining the bridge abutments.
In the Flathead River study, Boyd said they observed several erosion mechanisms, including severe scalloping of the bank, undercutting and saturated bank collapse.
When it comes to future river migration, Boyd said the maps outline future potential flood areas, but most would only occur under severe flooding circumstances.
One likely application for the maps would be to give landowners an idea where future problem areas might occur and how they could protect the banks from collapse.
Other possibilities could be using the maps for land sales, maintaining riparian areas and even helping farmers decide where to put their irrigation pivots.
In the future, there is also the possibility of setting ultimate boundaries for the river , which would show where to protect structures away from the current bank while letting the channel move around within the borders.
Already, there is seed money available for a voluntary river migration easement program, which would compensate landowners for allowing the river to take its natural course, Boyd said. Landowners would be able to use their land until the river hits a predetermined boundary, then they could armor it as they see fit.
The new maps are helpful because they provide landowners and government officials with a reminder to consider river movement in their future plans.
“Showing this stuff graphically to people, it’s very compelling,” Boyd said. “It’s a nice picture of the history of that place.”
In a following presentation, Dr. Mark Lorang of the Flathead Lake Biological Station showed the results of his bathymetric study, which mapped the depth and velocity of the river.
Before the Hungry Horse Dam was in place, the river stayed the same width when it moved, Lorang said. Post dam, with higher lake levels and full flow from June to September, the river levels go down slowly but build up quickly.
This leads to different rates of erosion along the banks. During his study, Lorang mapped out different erosion levels with a GPS system.
“This river is amazingly resilient, ecologically,” Lorang said. “It’s really healing itself in many areas.”
The study also used an acoustic Doppler velocity profiler – “a really fancy fish-finder,” Lorang said – to measure how deep the river is and the water’s velocity at different water levels.
He found that the Flathead River has surprisingly variable depths, even finding a 90-foot-deep hole in one bend. The results also showed that most of the river’s strength is forced downward, which makes avulsion flooding unlikely in most place.
Copies of the channel migration study and the river maps are available at the Flathead Lakers’ website at www.flatheadrivertolake.org/index.php/news-and-events.